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These messages are just sign of ssh bots bruteforcing your host. This happens to almost every machine connected to internet. (At least that is my experience). I strongly recommend you install sshguard, which can blacklist IPs, when it detects bruteforce attempt.

Change the port you run ssh on, that will stop 99.99% of hack attempts. It's in /etc/ssh/sshdconfig... change "Port 22" to "Port <random>", where <random> is a port you pick randomly between 10000 and 65000, say. Then you can use "ssh -p <random>" from your clients.

Soon you will decide it's totally crap typing that at your clients instead of just "ssh", so you put it in ~/.ssh/config like this: